This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, onto a substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, this invention pertains to microlithography performed using so-called xe2x80x9csoft X-raysxe2x80x9d (SXR), also known as xe2x80x9cextreme ultravioletxe2x80x9d (EUV) radiation (these two terms are used interchangeably herein). Even more specifically, the invention pertains to multilayer mirrors that are reflective to soft X-rays, as used in microlithography apparatus employing SXR (EUV) radiation, and to image formation in the SXR (EUV) band.
In recent years in response to the ever-increasing miniaturization and densification of microelectronic circuit elements as used in, for example, semiconductor integrated circuits, microlithography apparatus and methods have been developed that employ a soft X-ray beam as an energy beam in order to achieve better resolution of circuit elements. Soft X-ray (SXR) radiation has a wavelength generally within the range of 11 nm to 14 nm, which is significantly shorter than the radiation used to date (up to deep ultraviolet) in optical microlithography. In other words, microlithography technology used to date is compromised by diffraction limits, which prevent obtaining ever increasing resolution (e.g., see Tichenor et al., Proc. SPIE 2437:292, 1995).
SXR microlithography (also termed herein xe2x80x9cextreme ultraviolet,xe2x80x9d or EUV microlithography) offers prospects of attaining better resolution of circuit features than current microlithography technology. Specifically, EUV microlithography is hailed as the xe2x80x9cmicrolithography of the future,xe2x80x9d capable of achieving resolutions of about 70 nm and smaller, which cannot be achieved using so-called xe2x80x9coptical microlithographyxe2x80x9d (performed using a wavelength of about 190 nm or more).
With EUV wavelengths, the refractive index of substances is extremely close to unity. As a result, conventional optical elements for achieving refraction and/or reflection of optical wavelengths cannot be used. Instead, grazing-incidence mirrors or multilayer-film mirrors typically are used. Grazing-incidence mirrors achieve total reflection with a refractive index of slightly less than unity, and multilayer-film mirrors achieve a high overall reflectivity by passing weakly reflected light through multiple phase-matched convolutions. For example, a reflectivity of 67.5% can be obtained of a normal incident beam having a wavelength of about 13.4 nm using a reflective mirror comprising a Mo/Si multilayer film, in which molybdenum (Mo) layers and silicon (Si) layers are alternately laminated. A reflectivity of 70.2% can be obtained of a directly incident beam having a wavelength of about 11.3 nm using a reflective mirror comprising a Mo/Be multilayer film, in which Mo layers and beryllium (Be) layers are alternately laminated. E.g., see Montcalm, Proc. SPIE 3331:42, 1998.
Conventional soft X-ray microlithography apparatus comprise a soft X-ray source, an illumination-optical system, a mask stage, an imaging-optical (projection-optical) system, and a substrate stage. The SXR source can be a laser-plasma source, a discharge-plasma source, or a synchrotron-radiation source. The illumination-optical system comprises grazing-incidence mirrors each having a respective reflective surface that reflects SXR radiation that is obliquely incident to the reflective surface, multilayer-film mirrors each having a reflective surface are formed by a multilayer film, and a filter that transmits only SXR radiation of a specified wavelength. Thus, the mask is illuminated by SXR radiation having a desired wavelength.
Since no known substances are transparent to SXR radiation, the mask is a so-called xe2x80x9creflective maskxe2x80x9d rather than a conventional transmission-type mask. The imaging-optical system comprises multiple multilayer-film mirrors, and is configured to form an image, of the irradiated region of the mask, on the substrate (e.g., semiconductor wafer) to which a layer of a suitable resist has been applied. Thus, the image is transferred to the layer of resist. Since SXR radiation is absorbed and attenuated by the atmosphere, the SXR light path in the microlithography apparatus normally is maintained at a certain vacuum (e.g., 1xc3x9710xe2x88x925 Torr or less).
As noted above, the imaging-optical system comprises multiple multilayer-film mirrors. Since the reflectivity of a multilayer-film mirror is not 100 percent, the imaging-optical system desirably consists of as few such mirrors as possible to minimize light loss. Thus far, imaging-optical systems comprising four multilayer-film mirrors (e.g., Jewell and Thompson, U.S. Pat. No. 5,315,629; and Jewell, U.S. Pat. No. 5,063,586) and six multilayer-film mirrors (e.g., Williamson, U.S. Pat. No. 5,815,310) have been reported. Unlike refractive optical systems through which a light flux propagates in one direction, reflective optical systems are characterized by the doubling back of the light flux on itself within the optical system. Hence, it is difficult to obtain a large numerical aperture (NA) due to restrictions such as avoiding eclipsing the light flux with the mirrors.
Whereas a NA of no more than about 0.15 can be obtained in a four-mirror imaging-optical system, it is possible for a six-mirror optical system to have an even greater NA. Normally, an even number of mirrors is used in the imaging-optical system so that the mask stage and the substrate stage can be situated on opposite sides of the optical system. Since the imaging-optical system must correct aberrations using a limited number of surfaces, each of the mirrors typically has an aspherical profile, and a ring-field imaging scheme is used in which aberrations are corrected only in the proximity of a desired lateral displacement from the optical axis. To transfer the entire mask pattern onto the substrate, exposure is performed while scanning the mask stage and the substrate stage at respective velocities that differ from each other according to the magnification ratio of the imaging-optical system.
Imaging-optical systems, as discussed above, for use in SXR microlithography apparatus are so-called xe2x80x9cdiffraction-limitedxe2x80x9d optical systems. These optical systems cannot achieve the performance levels for which they were designed unless wavefront aberrations can be minimized adequately. A guideline for tolerances of wavefront aberration in a diffraction-limited optical system is Marechal""s standard, in which the root-mean-square (RMS) departure of the wavefront from a reference sphere that is centered on the diffraction focus does not exceed xcex/14, wherein xcex is wavelength. Born and Wolf, Principles of Optics, 7th edition, Cambridge University Press, 1999, p. 528. These are the conditions for obtaining 80% or more of the Strehl intensity (the ratio of maximum point-image intensities in an optical system with aberrations to maximum point-image intensities in an optical system with no aberrations).
In the EUV microlithography techniques currently under vigorous research and development, exposure xe2x80x9clightxe2x80x9d is used having a wavelength primarily in the range of 13 nm to 11 nm. With respect to wavefront error (WFE) in an optical system, the form error (FE) allowed for each individual mirror is given by Equation (1):
FE=(WFE)/2/{square root over (n)}(RMS value)xe2x80x83xe2x80x83(1)
In Equation (1), xe2x80x9cnxe2x80x9d is the number of mirrors that make up the optical system, and WFE is divided by 2 because wavefront error is double the form error. This is because both incident light and reflected light in a reflective optical system are subject to the effects of each respective form error.
The form error (FE) allowed for each individual mirror in a diffraction-limited optical system is given by Equation (2), relative to wavelength xcex and number of mirrors n:
FE=xcex/28/{square root over (n)}(RMS value)xe2x80x83xe2x80x83(2)
In the case of a 4-mirror imaging-optical system, this value is 0.23 nm (RMS) at a wavelength of 13 nm and 0.20 nm (RMS) at a wavelength of 11 nm. In the case of an optical system comprising 6 mirrors, this value is 0.19 nm (RMS) at a wavelength of 13 nm and 0.16 nm (RMS) at a wavelength of 11 nm.
Unfortunately, a high-precision aspherical mirror satisfying the foregoing is extremely difficult to manufacture. This is the main reason why a practical SXR microlithography apparatus has not been realized yet. The fabrication accuracy achievable to date for an aspherical mirror is about 0.4 to 0.5 nm (RMS). Gwyn, Extreme Ultraviolet Lithography White Paper, EUV LLC, 1998, p. 17. Consequently, fabrication and design techniques for aspherical surfaces used in mirrors in imaging optical systems must be improved substantially in order to achieve a practical SXR microlithography apparatus that exhibits higher resolution than obtainable with current optical lithography.
In view of the shortcomings of the prior art as summarized above, an object of this invention is to provide SXR microlithography apparatus and methods that can achieve substantially improved resolution of pattern elements on a substrate than achievable using conventional SXR microlithography technology.
To such end, and according to a first aspect of the invention, microlithography apparatus are provided for forming an image, on a resist-coated substrate, of a pattern defined by a mask. An embodiment of such an apparatus comprises an illumination-optical system and an imaging-optical system (the latter also termed a xe2x80x9cprojection-optical systemxe2x80x9d). The illumination-optical system is situated and configured to direct an illumination light, having a wavelength within a range of 20 nm to 50 nm, from a source of the illumination light to a mask. The imaging-optical system comprises multiple reflective mirrors having at least one aspherical surficial profile. The imaging-optical system is situated and configured to direct an imaging light, propagating from the mask, to a substrate so as to achieve a pattern-element resolution of 71 nm or finer.
The apparatus can include an illumination-light source situated and configured to produce the illumination light and to direct the illumination light to the illumination-optical system. The illumination-light source can be any of the following: a laser-plasma X-ray source, a discharge-plasma light source, a synchrotron-radiation source, and an X-ray laser.
The mask typically is a reflective mask. With such a mask, the illumination-optical system is configured to direct the illumination light to the reflective mask, and the imaging-optical system is configured to receive the imaging light, formed by reflection of the illumination light from the mask, and to direct the imaging light to the substrate.
In a first example, the illumination light has a wavelength within a range of 20 nm to 22 nm. In such an instance, the imaging-optical system desirably has a numerical aperture of at least 0.15.
In a second example, the illumination light has a wavelength within a range of 20 nm to 36 nm. In such an instance, the imaging-optical system has a numerical aperture of at least 0.25.
In a third example, the illumination light has a wavelength within a range of 20 nm to 43 nm. In such an instance, the imaging-optical system has a numerical aperture of at least 0.3.
In a fourth example, the illumination light has a wavelength within a range of 20 nm to 50 nm. In such an instance, the imaging-optical system has a numerical aperture of at least 0.35.
The imaging-optical system can comprise multiple multilayer-film reflective mirrors each having, as a respective reflective surface, an aspherical surface coated with a respective multilayer film. The multilayer film comprises multiple sets of alternating layers of a first material having a refractive index that is as different as possible from the refractive index of a vacuum and of a second material having a refractive index that is as close as possible to the refractive index of a vacuum. The first material can be any of the following: boron, ruthenium, manganese, yttrium, zirconium, niobium, alloys of these elements, and compounds of these elements. The second material can be any of the following: lithium, magnesium, aluminum, alloys of these elements, and compounds of these elements.
Another embodiment of a microlithography apparatus includes an illumination-optical system situated and configured to direct an illumination light, having a wavelength within a range of 20 nm to 50 nm, from a source of the illumination light to a mask. The apparatus also includes an imaging-optical system having a numerical aperture of at least 0.15. In this embodiment, the imaging-optical system desirably comprises multiple multilayer-film reflective mirrors each having, as a respective reflective surface, an aspherical surface coated with a respective multilayer film. The multilayer film comprises multiple sets of alternating layers of a first material having a refractive index that is greatly different (as defined above) from the refractive index of a vacuum and of a second material having a refractive index that is slightly different (as defined above) from the refractive index of a vacuum. Specific examples of first and second materials are as summarized above. The apparatus can include an illumination-light source situated and configured to produce the illumination light and to direct the illumination light to the illumination-optical system. Specific examples of such illumination-light sources are as summarized above.
According to another aspect of the invention, multilayer-film reflective mirrors are provided for use in reflecting soft X-ray light. An embodiment of such a mirror comprises a mirror substrate including a reflective aspherical surface coated with a multilayer film. The multilayer film comprises multiple sets of alternating layers of a first material having a refractive index that is greatly different (as defined above) from the refractive index of a vacuum and of a second material having a refractive index that is slightly different (as defined above) from the refractive index of a vacuum. Each set consists of at least one layer of the first material and one layer of the second material. Example first and second materials are as summarized above. Desirably, the multilayer film comprises at least 20 sets of alternating layers (e.g., 20-40 sets).
By appropriately selecting the first and second materials and forming the multilayer films as summarized above, multilayer-film mirrors can be formed that reflect exposure light having a wavelength within a range of 20 nm to 50 nm with high reflectivity. By employing such mirrors in an imaging-optical system of a SXR microlithography apparatus, the required exposure time for wafers can be shortened using 20-nm to 50-nm soft X-rays.
According to another aspect of the invention, methods are provided for manufacturing a microelectronic device. In an embodiment of such a method, a soft X-ray beam (having a wavelength in the range of 20 nm to 50 nm) is directed to a mask defining a pattern. Soft X-ray light from the mask is projected onto a resist-coated wafer so as to form an image of the pattern on the wafer at a resolution of 71 nm or finer.
In another embodiment of a method for manufacturing a microelectronic device, a soft-X-ray beam (having a wavelength in the range of 20 nm to 50 nm) is directed to a mask defining a pattern. Soft X-ray light from the mask is passed through an imaging-optical system (having a numerical aperture of at least 0.15) onto a resist-coated wafer so as to form an image of the pattern on the wafer.
In yet another embodiment of a method for manufacturing a microelectronic device, an imaging-optical system is provided that is configured to project a soft X-ray beam, having a wavelength in a range of 20 nm to 50 nm and propagating from a pattern-defining mask, to form an image of the pattern on a substrate. The imaging-optical system is configured to have a numerical aperture of at least 0.15. After previously applying a patterned layer to a substrate, the substrate is polished. A layer of a resist then is applied to the polished substrate. A soft X-ray illumination beam is directed to the mask. Soft X-ray light from the mask is passed through the imaging-optical system onto the resist-coated substrate so as to form an image of the pattern on the substrate, wherein the image overlays the previously applied patterned layer on the substrate. This embodiment is especially useful in instances in which overlay exposure is being performed on a wafer in which circuit patterns have already been formed. By polishing and then exposing the already exposed wafer, the pattern defined by the mask can be transferred accurately to the wafer even if the depth of focus imaging-optical system is small.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.